The present invention relates to electronic circuits and methods of operation thereof, and more particularly, to filter circuits and methods of operation thereof.
Traditional superheterodyne radio receiver designs typically use passive intermediate frequency (IF) filters. In many advanced receiver circuit designs, in particular, receiver designs implemented using application specific integrated circuits (ASICs), passive filters have been replaced with active filters that typically are more easily implemented in an ASIC. An active filter structure commonly used in ASIC-based IF circuits is the so-called xe2x80x9cGm-Cxe2x80x9d filter, which uses transconductance amplifiers and capacitor gyrators to simulate inductors.
The transconductance and capacitance of an integrated circuit Gm-C IF filter can vary with fabrication process conditions. Variations in temperature and power supply voltage can also cause variation of the transfer characteristics of the filter, which may require periodic re-tuning to maintain the passband shape and center frequency. Traditional approaches to tuning Gm-C filters include master-slave tuning techniques, as described in xe2x80x9cThe Problem of On-Chip Automatic Tuning in Continuous-Time Integrated Filters,xe2x80x9d Schauman et al., IEEE Proceedings of ISCAS, pp. 106-109, 1989. According to this type of approach, a duplicate (master) filter is formed on the same chip with the IF filter. A reference signal is applied to the duplicate filter to determine its transfer characteristics, and the IF (slave) filter is adjusted accordingly based on the assumption that the characteristics of the master and slave filters are closely matched.
The conventional master/slave tuning technique can be disadvantageous for a number of reasons. To achieve accurate tuning, the components of the master and slave filters generally must be very closely matched. In addition, the additional master filter generally consumes chip area that could be better utilized for other circuitry. Accordingly, there is a need for improved tuning methods and apparatus for tuning receiver filter circuits.
In light of the foregoing, it is an object of the present invention to provide improved methods and apparatus for tuning bandpass filters such as the IF filters employed in a radio communications circuit.
It is another object of the present invention to provide improved methods and apparatus for tuning bandpass filters that can be implemented using fewer components than conventional techniques.
It is yet another object of the present invention to provide improved methods and apparatus for tuning an IF filter in a radio receiver circuit which can be implemented using existing components of the radio receiver circuit.
These and other objects, features and advantages are provided according to the present invention by methods and apparatus in which a bandpass filter, such as an IF filter of a radio receiver circuit, is tuned by providing a signal having a substantially uniform spectral distribution to the input of the filter, determining an average frequency for a limited signal produced by a limiter following the filter, and adjusting the filter based on the determined average frequency. A sampling interval may be determined based on the desired center frequency for the filter and a desired confidence interval, and the limited signal sampled for the desired sampling interval to achieve a plurality of samples from which an average frequency can be determined. The samples may be obtained by processing the limited signal using a detector included in the receiver circuit. The filter may be Gm-C filter that has a transconductance which is adjustable responsive to a control signal applied to the filter. The control signal may be produced based on the determined average frequency. The filter may be adjusted based on the determined average frequency until a center frequency is achieved which is within a predetermined range with respect to a desired center frequency.
The present invention arises from the realization that in applying a spectrally uniform signal to the input of the bandpass filter, it can be expected that the limited signal produced by the limiter will have a spectrum which has an approximately Gaussian distribution around the center frequency of the bandpass filter. Accordingly, by sampling the limited signal and determining an average frequency thereof, an estimate of the center frequency of the bandpass filter can be obtained with a desired resolution and confidence level. Because the limiter and detector are components normally present in the radio receiver circuit, additional circuitry, such as a master reference filter matched to the bandpass filter, is not required. As the bandpass filter can otherwise be designed such that the passband shape is resistant to variations due to temperature and power supply fluctuations, tuning of the center frequency of the bandpass filter can be performed upon power-up of the receiver circuit.
In particular, according to the present invention, a signal having a substantially uniform spectral distribution is provided at the input of a bandpass filter to thereby produce an output signal at the output of the bandpass filter. The output signal is processed in a limiter to produce a limited signal. An average frequency of the limited signal is determined, and the bandpass filter is adjusted based on the determined average frequency. According to one embodiment of the present invention, the bandpass filter comprises a Gm-C filter having a transconductance, and the filter is adjusted by adjusting the transconductance of the Gm-C filter based on the determined average frequency.
According to another aspect of the present invention, a desired center frequency for the bandpass filter is identified. A resolution and a desired confidence interval are also identified. The number of samples of the limited signal needed to achieve the identified desired resolution and confidence interval is determined based on the identified desired center frequency. An average frequency is determined by sampling the limited signal to obtain a plurality of samples, the number of the plurality of samples being at least as great as the determined number of samples of the limited signal to achieve the identified desired resolution and confidence level, and determining the average frequency from the plurality of samples. The bandpass filter is adjusted based on the determined average frequency to achieve a center frequency for the bandpass filter that is within a predetermined range with respect to the desired center frequency. The sampling, determining, and adjusting steps may be repeatedly performed until the average frequency is within a predetermined range with respect to the desired center frequency.
According to another aspect of the present invention, an IF filter of a radio receiver circuit is tuned. The radio receiver circuit includes a limiter connected to an output of the IF filter. A signal having a substantially uniform spectral distribution is provided at an input of the IF filter to thereby produce an output signal at the output of the IF filter. The output signal is processed in the limiter to produce a limited signal. An average frequency of the limited signal is determined, and the IF filter is adjusted based on the determined average frequency. The IF filter may include a Gm-C filter having a transconductance, and adjustment of the filter may comprise adjusting the transconductance of the Gm-C filter according to the determined average frequency.
According to another aspect of the present invention, the IF filter has a passband within a range of frequencies. A noise signal having a substantially uniform spectral distribution within the range of frequencies is provided to the input of the IF filter. For example, the IF filter may be implemented in a circuit having a signal ground, and the radio receiver circuit may be configured to connect the input of the IF filter to one of a radio signal path or the signal ground. The noise signal may be provided by connecting the signal ground to the input of the IF filter. According to another related aspect, connection of the signal ground occurs in response to application of power to the radio receiver circuit. After adjustment of the IF filter, the input of the IF filter may be disconnected from the signal ground and the IF filter connected to a radio signal path.
An apparatus according to an embodiment of the present invention includes a bandpass filter including an input and an output. A signal generator provides a signal having a substantially uniform spectral distribution at the input of the bandpass filter to thereby produce an output signal at the output of the bandpass filter. A limiter is responsive to the bandpass filter and processes the output signal of the bandpass filter to produce a limited signal. Means are provided, responsive to the limiter, for determining an average frequency of the limited signal. Additional means are provided, responsive to the means for determining an average frequency, for adjusting the bandpass filter based on the determined average frequency.
In yet another embodiment according to the present invention, a radio receiver circuit includes an IF filter having an input and an output. Means are provided for providing a signal having a substantially uniform spectral distribution at the input of the IF filter to thereby produce an output signal at the output of the IF filter. A limiter is connected to the output of the IF filter, and limits the output signal to produce a limited signal. Means are responsive to the limiter for determining an average frequency of the limited signal. The means for determining an average frequency may include a detector, a digital signal processor (DSP) and a microcontroller as may be commonly found in a cellular radiotelephone receiver circuit, for example. Additional means are provided, responsive to the means for determining an average frequency, for adjusting the IF filter based on the determined average frequency. For example, the IF filter may include a Gm-C (transconductance) filter which is responsive to a control signal applied thereto, and the means for adjusting the IF filter may comprise means for generating the control signal. A receiver circuit that is tunable using commonly used receiver components is thereby provided.